In recent years, there has been a rapid advancement in the development of potable and cordless electronic apparatuses. With the development, there has been an increasing demand for small-size, light-weight, thin, and high energy density secondary batteries that serve as the power supply for these apparatuses.
In particular, since non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries and lithium polymer secondary batteries, have a high voltage and high energy density and can be made thin, the development thereof is on the increase.
For the positive electrodes of non-aqueous electrolyte secondary batteries, various kinds of lithium-containing composite oxides capable of absorbing and desorbing lithium by charging and discharging are generally used as active materials.
On the other hand, for the negative electrodes of non-aqueous electrolyte secondary batteries, there are ones containing metal lithium as an active material; and ones containing a metal compound or a carbon material capable of absorbing and desorbing lithium by charging and discharging. At present, carbon materials are often used for the negative electrodes.
Normally, on the surface of a negative electrode using a carbon material, an SEI (Solid Electrolyte Interface) film is formed by the reaction between an electrolyte and the negative electrode, and the chemical stability of the negative electrode is maintained by this film. However, during storage of the battery at high temperature, the film cracks easily, and there arises the problem that the negative electrode and the non-aqueous solvent in the electrolyte react with each other in the cracked portion and generate a gas.
In a lithium polymer secondary battery, in order to reduce the size, weight and thickness, a bag-shaped outer packaging material made of a laminated sheet comprising resin films and a metal foil sandwiched therebetween is often used in place of a metal case used for a lithium ion secondary battery. In this case, even when a small amount of a gas is generated, there will be problems, such as an increase in the thickness of the battery, leakage of the electrolyte and deterioration of the charge and discharge characteristics, due to an increase in the internal pressure of the battery.
Moreover, there is a problem that a carbon material easily separates from a negative electrode during charging of a battery. It is considered that this is caused by distortion of the carbon material due to a gas generated during an electrochemical reduction of a non-aqueous solvent at the interface between the carbon material in the negative electrode and the non-aqueous electrolyte.
Furthermore, when binders contained in a positive electrode and negative electrode are swollen with an electrolyte during charge/discharge cycle and storage of the battery, there arises the problem that the impedances of the electrodes increase and the battery capacity decreases gradually.